Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, such as silicon (Si), germanium (Ge), and gallium arsenide (GaAs). A field-effect transistor (FET) is a semiconductor device that includes three terminals: a gate, a source, and a drain. A FET uses an electric field applied by the gate to control the electrical conductivity of a channel through which charge carriers flow from the source to the drain. Some FETs have a fourth terminal called, the body or substrate, which serves to bias the transistor into operation. A metal-oxide-semiconductor FET (MOSFET) uses an insulator between the gate and the body of the transistor and MOSFETs can be used for amplifying or switching electronic signals. MOSFETs are commonly found in both digital and analog circuits. MOSFETs typically include side-wall spacers, referred to generally as spacers, on either side of the gate that help determine the channel length and help with replacement gate processes, for example. In some cases, portions of the source and/or drain regions of a transistor extend under the spacers (and in some cases, under the gate), and such portions are generally referred to as source/drain extensions or tips.
These and other features of the present embodiments will be understood better by reading the following detailed description, taken together with the figures herein described. In the drawings, each identical or nearly identical component that is illustrated in various figures may be represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. Furthermore, as will be appreciated, the figures are not necessarily drawn to scale or intended to limit the described embodiments to the specific configurations shown. For instance, while some figures generally indicate straight lines, right angles, and smooth surfaces, an actual implementation of the disclosed techniques may have less than perfect straight lines and right angles, and some features may have surface topography or otherwise be non-smooth, given real-world limitations of fabrication processes. In short, the figures are provided merely to show example structures.